THE INFLUENCE OF ANIMAL AGE AND SWARD HEIGHT ON THE HERBAGE INTAKE AND GRAZING BEHAVIOR OF CHAROLAIS CATTLE

Mature dry cows, 18-month-old heifers and 7-month-old female calves of the Charolais breed (eight animals of each age) were strip-grazed on moderate (15cm) or tall (52 and 32cm) cocksfoot swards over two 15-day experimental periods. The swards were leafy regrowths of (shout) and 8 (tall) weeks of age. Herbage in take was estimated by the alkane method, and grazing behaviour was visually recorded. Bite weight was lower, grazing time and biting rate greater, and organic matter intake of grass (OMIG) higher on shorter swards, the latter being related to a higher in vitro digestibility and nitrogen concentration. Average OMIG per kg live weight (M) was 15.2, 19.1 and 17.0 (s.e.d. 0.49)g in cows, heifers and calves respectively. The average allometric coefficient between OMIG and M was 0.88 (similar between swards), and close to that obtained with hay after the grazing trial. Faecal markers suggested a greater selectivity of calves compared with older cattle. Average bite weights (BW) were 310 (s.e. 40), 467 (s.e. 112) and 718 (s.e. 144)mg OM in calves, heifers and cows respectively. The allometric coefficient between BW and M was higher in shout swards than in tall ones. Biting rates increased and grazing limes tended to decrease with age of cattle.